A hybrid vehicle and an electric vehicle have been developed to reduce emission and improve fuel economy. In such a hybrid vehicle and an electric vehicle, the direct current voltage of a battery is changed into the alternating voltage by an inverter to drive a motor. JP-2008-5615A shows a system which avoids overheat of the inverter. In this system, the coolant is circulated between the inverter and the radiator. When the temperature of the inverter exceeds a predetermined threshold, the motor is controlled to reduce the output torque.
Since the hybrid vehicle has an engine and a motor, an engine cooling system and an inverter cooling system are necessary.
Moreover, a plug-in hybrid vehicle has been developed. In the plug-in hybrid vehicle, a motor driving period (EV mode) is longer than a normal hybrid vehicle. Thus, the motor should output higher power to improve the power performance in the EV mode. However, such a high power motor and inverter increase manufacturing cost of the vehicle.